In the gold mining industry, gold-bearing ore is mined and then subjected to steps to recover the gold. The gold may be present in the ore on a microscopic scale and/or steps may be taken to grind or otherwise comminute the ore so that the gold is in a form suitable for extraction. Concentrate may be used. Methods for the recovery of gold from ore or especially concentrate include use of pyrometallurgical processes. Alternatively, concentrate may be subjected to cyanide leaching, by contacting the gold ore concentrate with solutions of cyanide and oxidant in alkaline solution; acidic solutions of cyanide result in the formation of hydrogen cyanide which is extremely hazardous and thus acidic treatment of waste solids obtained after cyanide leaching is also to be avoided. Notwithstanding any recycling or other steps in the process, a consequence of such a process is that large volumes of toxic solids and liquids are produced, which must be disposed of in an environmentally-acceptable manner.
One method of recovery of gold from solution after cyanide leaching is use of activated carbon. A variety of methods may be used, as discussed in US patent application 2010/0296987 A1 of O. Hyvarinen et al, published 25 Nov. 2010. For example, gold is precipitated from solution onto carbon using a chloride-based solution, for example using a solution of Cu++ chloride and sodium chloride. Subsequently, the gold may be redissolved using cyanide and recovered by precipitation with zinc or by electrolysis.
U.S. Pat. No. 7,547,348 of S-E. Hultholm et al, issued 16 Jun. 2009, exemplifies the leaching of copper and nickel from a sulphidic composition containing Cu, Ni, Fe, S, Pd and 14 ppm of gold at a temperature of 90° C. and a redox potential of 500 mV with pure 25% hydrochloric acid; it is stated that PGMs start to dissolve at a higher redox potential. The resulting acidic leach solution, which contained copper and nickel in solution, was extracted with a known copper extractant, such as an oxime. Subsequently, the organic phase was washed with dilute sulphuric acid to remove iron and chloride residue. The copper-containing organic solution is then stripped. The leach residue from the process contains the PGMs and most of the sulphidic sulphur. The PGM concentrate that was ultimately obtained contained 2% gold and was stated to be suitable for sale or further refining.
U.S. Pat. No. 7,785,395 of J. Leppinen et al, issued 31 Aug. 2010, discloses leaching with Cu++ chloride and alkali metal chloride to effect dissolution of copper and some gold. After further processing, gold is recovered using activated carbon or by ion exchange. U.S. Pat. No. 7,799,114 of L. Haavanlammi et al, issued 21 Sep. 2010, discloses that gold contained in waste or intermediate fractions is leached by means of copper++ chloride, oxygen and alkali bromide under conditions in which the oxidation potential is a maximum of 650 mV and the pH a minimum of 0.5. The bromide is stated to accelerate the dissolution of gold. U.S. Pat. No. 7,776,135 of M. Hamalainen et al, issued 17 Aug. 2010 also discloses a process in which copper and gold are leached using a solution of copper++ chloride, sodium chloride and oxygen.
Alternative lixiviants to cyanide, including halide lixiviants, are discussed in a paper presented by M. G. Aylmore at the World Gold Conference 2011, 2-5 Oct. 2011, part of the Conference of Metallurgists, 41st Hydrometallurgy Meeting in Montreal. Halide leaching of gold using chlorine, bromine and iodine of gold was discussed. Alternative process use 5M hydrochloric acid under oxidizing conditions with chlorine or a chloride electrolyte (NaCl and NaBr with added chlorine/bromine complex (BrCl2−)). Leaching in regenerated hydrochloric acid is also known although details are stated to be vague. Several other oxidants e.g. oxygen or nitric acid dissolve gold in the presence of chloride; ferric chloride may also be used with chloride. It is stated that most of these alternative processes are restricted to high-grade concentrates and have been used as a pre-treatment process for leaching gold with cyanide. Lack of a suitable recovery process to match that from cyanide extraction has been the main limitation of the use of chloride processes. Thiosulphate has been used in the leaching of gold, especially on copper-gold and carbonaceous ores that give poor gold recoveries using cyanide. It is also know to use thiourea, thiosulphate and cyanide in elution of gold absorbed onto activated carbon or resin-packed fixed columns.
In other mining industries, processes have been developed to extract value metals e.g. nickel and titanium, with recycling of solutions used in the process and with substantially reduced environmental issues. One example is the use of solutions of hydrochloric acid and magnesium chloride in the extraction of titanium from titanium-bearing ores or concentrates at low temperatures, as disclosed in U.S. Pat. No. 7,803,336 of V.I. Lakshmanan, R. Sridhar et al, issued 28 Sep. 2010. U.S. Pat. No. 7,736,606 of V.I. Lakshmanan, R. Sridhar et al, issued 15 Jun. 2010, describes extraction of base metal sulphide ores with a lixiviant of a chloride, an oxidant and hydrochloric acid.
Processes that are effective for the recovery of gold from gold-bearing ores or concentrates and which have low or minimal environmental issues would be beneficial. Such processes should include potential for recycling of components used in the process so as to reduce environmental issues and for economic operation of the process. In addition, processes should be capable of separating gold from other value metals in leaching steps. There is a particular need for such processes in the gold mining industry.